These studies are concerned with the cell and molecular mechanisms involved in target cell lysis mediated by human lymphoid cells in vitro. These are complex reactions, but there appear to be three common steps; a) recognition - promoted by lymphocyte and target cell membrane interaction; which results in b) activation of the lymphocyte to become the effector cell; and c) lysis of the target cell, which can be either rapid or delayed. These studies have been predominantly directed toward understanding the mechanism(s) involved in cell lysis. Human lymphocytes, once activated, can secrete a family of protein in vitro which are cell-toxins. Physical-chemical studies reveal that LT molecules are proteins and can be separated on the basis of physical-chemical properties into four families, each with submembers. This situation appears to be the result of condensation of three monomer units to form dimers and tetramers. Antisera has been generated which will neutralize individual members or whole families of human LT molecules. Studies with purified IgG or Fab fragments provided definitive proof that these molecules are major lytic effectors in target cell lysis in vitro mediated by mitogen activated human lymphocytes. Initial data also suggests they participate in immune lytic reactions. LT molecules are present on the aggressor lymphocyte membrane, and the lymphoid cell may employ LT to cause cell lysis by two mechanisms, direct contact or secretion. Finally, initial studies have demonstrated that alpha and beta-LT molecules are present in the sera of human patients with various forms of ongoing cell mediated immune tissue destructive reactions.